1. Field of the Invention
The present invention relates to a bipolar transistor and, more particularly, to a polysilicon-edge, low-power, high-frequency bipolar transistor and a method of forming the transistor.
2. Description of the Related Art
A high-frequency bipolar transistor is a device that can turn off and on again fast enough to respond to a high-frequency signal without distorting the wave shape of the signal. A low-power high-frequency bipolar transistor is a device that consumes very little power in responding to the high-frequency signal. Low-power high-frequency bipolar transistors are used in wireless applications, and are finding uses in emerging optical networking applications.
FIG. 1 shows a cross-sectional diagram that illustrates a portion of a prior-art, low-power high-frequency bipolar transistor 100. As shown in FIG. 1, transistor 100 includes a collector layer 110, a base layer 112 that is formed on collector layer 110, and a field oxide region FOX that adjoins layer 112. In addition, transistor 100 includes a thin oxide layer 114 that is formed on a portion of base layer 112 and the field oxide region FOX, and a n+ extrinsic emitter 116 that is formed on thin oxide layer 114.
As further shown in FIG. 1, transistor 100 also includes an n+ emitter region 118 that is formed in base layer 112, and an n+ poly ridge 120 that is connected to extrinsic emitter 116 and n+ emitter region 118. Extrinsic emitter 116, emitter region 118, and poly ridge 120 form the emitter of the transistor.
Transistor 100 additionally includes a silicided base contact 122 that is formed on base layer 112, and a silicided emitter contact 124 that is formed on extrinsic emitter 116. In addition, an oxide spacer 126 is formed on base layer 112 between poly ridge 120 and base contact 122.
During fabrication, emitter region 118 is formed from dopants diffusing from poly ridge 120 into base layer 112. As a result, a very small base-to-emitter junction results. A small base-to-emitter junction reduces the base-to-emitter capacitance which, in turn, allows low-power high-frequency operation.
One drawback of transistor 100, however, is that transistor 100 has a large base-to-collector capacitance which, in turn, limits the operation of the transistor. Thus, there is a need for a low-power high-frequency bipolar transistor with a reduced base-to-emitter capacitance and base-to-collector capacitance.
The present invention provides a low-power high-frequency bipolar transistor that reduces the base resistance, the base-to-emitter capacitance, and the base-to-collector capacitance. Thus, the present invention provides the advantages of the prior-art transistor while reducing the high base-to-collector capacitance of the prior-art bipolar transistor.
The bipolar transistor of the present invention is formed on a wafer that has a buried layer and a first epitaxial layer of a first conductivity type. The first epitaxial layer is formed over the buried layer and has a smaller dopant concentration than the buried layer.
The bipolar transistor has an intrinsic base region of a second conductivity type that is formed on the first epitaxial layer. The intrinsic base region has a first side wall, a second side wall, and a top surface that is connected to the first side wall via a first notch and to the second side wall via a second notch.
The bipolar transistor also has a layer of isolation material that is formed on the first epitaxial layer. The layer of isolation material contacts the first side wall and the second side wall of the intrinsic base region, and is formed over the first notch and the second notch. In addition, the bipolar transistor includes an intrinsic emitter region that is formed in the intrinsic base region, an extrinsic base that is formed on the layer of isolation material, and an extrinsic emitter that is formed on the layer of isolation material.
The bipolar transistor further includes a conductive base spacer that is connected to the extrinsic base and the intrinsic base region, and a conductive emitter spacer that is connected to the extrinsic emitter and the intrinsic base region. The conductive base spacer is formed over the first notch, while the conductive emitter spacer is formed over the second notch.
The present invention also includes a method for forming a low-power high-frequency bipolar transistor. The bipolar transistor is formed on a wafer that has a buried layer and a first epitaxial layer of a first conductivity type. The first epitaxial layer is formed over the buried layer and has a smaller dopant concentration than the buried layer.
The method of the present invention begins by forming a layer of isolation material on the first epitaxial layer, and forming an extrinsic base and an extrinsic emitter on the layer of isolation material. The extrinsic base, which is spaced apart from the extrinsic emitter, has a second conductivity type while the extrinsic emitter has the first conductivity type.
The method also includes the step of etching the layer of isolation material to form an opening in the layer of isolation material. The opening is between the extrinsic base and the extrinsic emitter, and exposes a surface of the first epitaxial layer. The method further includes the steps of forming a first intrinsic base region on the first epitaxial layer in the first opening, and forming an isolation region on the first intrinsic base region in the first opening.
The method additionally includes the step of forming a sacrificial material on the isolation region over the first intrinsic base region in the first opening. The method further includes the steps of etching the isolation region, and removing the sacrificial material after the isolation region has been etched.
In addition, a base spacer is formed that contacts the extrinsic base and the layer of isolation material formed on the first intrinsic base region, and an emitter spacer is formed that contacts the extrinsic emitter and the layer of isolation material on the first intrinsic base region. The base spacer is spaced apart from the emitter spacer.
The method also includes the step of etching the layer of isolation material to form a gap between the base spacer and the first intrinsic base region, and the emitter spacer and the first intrinsic base region. The method further includes the step of forming a second intrinsic base region to fill up the gap so that the second intrinsic base region contacts the first intrinsic base region, the base spacer, and the emitter spacer.
A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings that set forth an illustrative embodiment in which the principles of the invention are utilized.